I found below photos of size comparisons to help a little, but since I dont have the resources to visit a shop that sells a variety of different styles I cant get one in my hand to see which is more comfortable to grip

The reason you are having trouble finding information about the relative sizes of standardized nib units is that standardized nib units - units that would then be interchangeable among different makes and models of fountain pens - don't really exist.

Nib tip sizes, which then control the fineness or thickness of the writing line, are relatively standardized (although Japanese nibs, for example, generally run about one size finer than European or American ones). Thus if you specify a Fine or Medium nib, you know approximately how it will write. However the design of the nib itself varies from maker to maker and even model to model -- and that include the overall diameter or girth and weight.

I think what you would have to do is contact your preferred maker and ask him for details about the units he uses.

Those are kit pens. If you want a truly custom pen you should contact Chris at AppleMan Pen Creations; just Google and you'll find his site. When you refer to "nib housing" I think you mean "section", which is where the pen is gripped.